1. Field of the Invention
This invention generally relates to integrated circuit (IC) and solar cell fabrication and, more particularly, to a photovoltaic device using a conductive nanowire array electrode.
2. Description of the Related Art
A photovoltaic (PV) cell, which is often referred to as a solar cell, is most often a semiconductor device with a large-area p-n junction diode. In the presence of sunlight, the PV is capable of generating electrical energy. This conversion is called the photovoltaic effect. Efficiency is a known problem associated with solar cells. Efficiency is the ratio of the electric power output to the light power input. Solar radiation has an approximate maximum value of 1000 watts per square-meter (W/m2). Solar cell efficiencies vary between 6%, using amorphous silicon materials, and 30% or even higher efficiencies, using experimental materials.
The most common solar cell material is crystalline silicon. Single-crystal cells have an efficiency on the order of 14-20%, and are expensive, because they are cut from cylindrical ingots. Polycrystalline cells are made from cast ingots. While cheaper and more easily formed into desired shapes, these cells are less efficient. Nanocrystalline structures are inefficient, but easier to make. Self supporting wafers, of whatever crystalline structure are soldered together to form a module.
With thin-film approaches, the entire module substrate is coated and etched to differentiate individual cells. Amorphous silicon films are fabricated using chemical vapor deposition (CVD) techniques, typically plasma-enhanced (PECVD). These cells have low efficiencies of around 8%. CIS, which stands for general chalcogenide films of Cu(InxGa1-x)(SexS1-x)2, is a thin-film that can achieve an 11% efficiency at a relatively high manufacturing cost.
As described above, most solar cells are Si based. Since Si is a group IV atom, each atom has 4 valence electrons in its outer shell. Silicon atoms can covalently bond to other silicon atoms to form a solid. There are two basic types of solid silicon, amorphous and crystalline. As a solid semiconductor, there are certain bands of energies which the Si electrons are allowed to have, and other energies between these bands which are forbidden. These forbidden energies are called the bandgap.
At room temperature, pure silicon is a poor electrical conductor, as Fermi level lies in the bandgap. To improve conductivity, Si is doped with very small amounts of atoms from either group III or group V of the periodic table. These dopant atoms take the place of the silicon atoms in the crystal lattice, and bond with their neighbouring Si atoms in almost the same way as other Si atoms do. However, because group III atoms have only 3 valence electrons, and group V atoms have 5 valence electrons, there is either one too few, or one too many electrons to satisfy the four covalent bonds around each atom. Since these extra electrons, or lack of electrons (known as “holes”) are not involved in the covalent bonds of the crystal lattice, they are free to move around within the solid. If silicon is doped with group III atoms, such as aluminum or gallium, it becomes a p-type silicon because the majority charge carriers (holes) carry a positive charge, while silicon doped with group V atoms, such as phosphorus or arsenic, becomes n-type silicon because the majority charge carriers (electrons) are negative.
The absorption of photons creates electron-hole pairs, which diffuse to the electrical contacts and can be extracted as electrical power. When a photon of light hits a piece of silicon, one of two things can happen. First, the photon can pass straight through the silicon, which is likely if the energy of the photon is lower than the Si bandgap energy. Alternately, the photon is absorbed by the silicon, which is likely if the photon energy is greater than the bandgap energy of silicon. When a photon is absorbed, its energy is given to an electron in the crystal lattice. Usually this electron is in the valence band, and is tightly bound in covalent bonds between neighbouring atoms, and hence unable to move far. The energy given to it by the photon “excites” it into the conduction band, where it is free to move around within the semiconductor. The covalent bond to which the electron was previously bound now has a hole, as a result of losing an electron. Bonded electrons from neighboring atoms can move into this hole, leaving a hole behind them. In this manner, a hole can be said to move through the lattice. Alternately stated, the photons absorbed by the Si create mobile electron-hole pairs.
A typical solar cell includes a layer of n-type Si adjacent a layer of p-type Si. As explained above, electrons diffuse from the region of high electron concentration, which is the n-type side of the junction, into the region of low electron concentration, which is the p-type side of the junction. When the electrons diffuse across the p-n junction, they recombine with holes on the p-type side. As electrons from donor atoms on the n-type side of the junction cross into the p-type side, positively charged group V donor atoms nuclei make the n-type side of the junction positively charged. Simultaneously, as electrons fill the holes on the p-type side of the junction, the group III acceptor atoms create an excess of negative charge on the p-type side of the junction. This imbalance of charge across the p-n junction sets up an electric field which opposes further diffusion of charge carriers across the junction.
This region in which the electrons have diffused is called the depletion region or space charge region, as there are no mobile charge carriers. The electric field which is set up across the p-n junction creates a diode, which passes current in a single direction. That is, electrons flow from the n-type side into the p-type side, and holes flow from the p-type side to the n-type side.
However, if a photon-generated electron-hole pair is created within a minority carrier diffusion length of the junction, then current will flow across the junction. That is, the electric field at the junction will either sweep an electron to the n-type side, or a hole to the p-type side.
The invention of conductive polymers may lead to the development of much cheaper cells that are based on inexpensive plastics, rather than semiconductor silicon. However, organic solar cells suffer from degradation upon exposure to UV light and have limited lifetimes.
Cadmium telluride (CdTe) and copper indium gallium selenide (CIGS) can be used to make efficient solar cells. Unlike a p-n junction Si cell, these cells are best described by a more complex heterojunction model.
Polymer or organic solar cells are built from ultra thin layers organic semiconductors such as polyphenylene vinylene and fullerene. The p/n junction model only partially describes the cell operation. Other functions, such as electron hopping, also contribute to the cell operation. While these cells are cheaper to make than Si or inorganic cells, their efficiencies are low and sensitivity to the environment is high. However, the technology has already been applied commercially to organic LEDs and organic (polymer) displays.
FIG. 1 is a partial cross-sectional view of a bulk heterostructure with phase separation (prior art). In order to improve efficiency, a third generation organic photovoltaic (OPV) structure has been proposed using a bulk heterojunction—by casting solutions containing a blend of two organic films that are coated onto a substrate, as shown in FIG. 1. The problem with this kind of structure is the disordered nanostructures, in that the two phases created are separated by too large of a length scale. Consequently, some of the generated excitons are not able to diffuse to an interface to be dissociated before they decay. In other cases, the phases have “dead ends” that prevent charge carriers from reaching an electrode.
FIG. 2 is a partial cross-sectional view of an ordered bulk heterojunction solar cell using flat bottom and top electrodes, and a conjugated polymer to fill the pores (prior art). To address the phase separation problem, an ordered heterojunction has been proposed. One of the most attractive approaches is to use a block copolymer that self assembles to form an array of cylinders, oriented perpendicular to the substrate. Unfortunately this structure is very hard to make and is not commercially feasible.
Another approach is to use an organic-inorganic composite that combines conjugated polymer with a nanostructured, large bandgap inorganic semiconductor such as TiO2, CdS, or ZnO. The nanostructured inorganic semiconductor can be a nanowire or nanorod array grown on a flat conductive electrode. The conjugated polymer fills the pores. A flat transparent conductive top electrode is then deposited. Catalysts or seed layers are usually deposited on the bottom electrode to enhance the semiconductor array formation. Sometimes the formation temperature of the semiconductor array is high, preventing the use of the process with cheap glass or plastic substrates. Thus, it is difficult to grow a semiconductor nanowire or nanorod array overlying a flat-surface electrode.
It would be advantageous if a conductive nanorod array could be coated with semiconductor material using a thin-film process, instead of growing semiconductor nanowires from an electrode.